Irish grid reference system

The Irish grid reference system is a system of geographic grid references commonly used in Ireland (both Northern Ireland and the Republic of Ireland). The Irish grid overlaps the British grid.

Usage

In general, neither Ireland nor Great Britain uses latitude or longitude in describing internal geographic locations. Instead grid reference systems are in common usage.

The national grid referencing system was devised by the Ordnance Survey, and is heavily used in their survey data, and in maps (whether published by the Ordnance Survey of Ireland, the Ordnance Survey of Northern Ireland or commercial map producers) based on those surveys. Additionally grid references are commonly quoted in other publications and data sources, such as guide books or government planning documents.

The Ordnance Survey of Ireland has implemented a new coordinate system for Ireland called Irish Transverse Mercator, or ITM, which will initially run in parallel with the existing Irish grid system.

Grid letters

The area of Ireland is divided into 20 squares, measuring 100km by 100km, each identified by a single letter.

Eastings and northings

Within each square, eastings and northings from the origin (south west corner) of the square are given numerically. For example, G0305 means 'square G, 3 km east, 5 km north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a 1 km square) through to five (for a 1 m square); the most common usage is the "six figure grid reference", employing three digits in each coordinate to determine a 100 m square.

Summary parameters of the Irish Grid coordinate system:

:Datum: OSGB1936 Modified,:Map projection: Transverse Mercator,:Latitude of Origin: 53°30'00 N,:Longitude of Origin: 8°00'00 W,:Scale Factor: 1.000 035,:False Easting: 200000 m,:False Northing: 250000 m

Examples

* [http://getamap.ordnancesurvey.co.uk/getamap/frames.htm?mapAction=gaz&gazName=g&gazString=O008741 O 008 741] (Newgrange Neolithic tomb, County Meath on OSNI 1:250000 mapping from OrdnanceSurvey.co.uk Get-a-Map).
* [http://getamap.ordnancesurvey.co.uk/getamap/frames.htm?mapAction=gaz&gazName=g&gazString=C948454 C 948 454] (Giant's Causeway, County Antrim on OSNI 1:250000 mapping "ibid").

External links

* [http://www.osi.ie/GetAttachment.aspx?id=b2bd07a6-858e-4eb1-9d09-25917f0c713a "The Irish Grid - A Description of the Co-ordinate Reference System used in Ireland"] -PDF (358kB) on the Irish Grid from OSi.
* [http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa PDF (221KB) detailing GPS to Irish Grid conversions]
* [http://www.osni.gov.uk/2.1_the_irish_grid.pdf A detailed (42 page) PDF file including history and map of the Irish grid and its links to Britain]
* [http://www.asprs.org/resources/grids/03-99-ireland.pdf A two-page article for the American Society for Photogrammetry and Remote Sensing summarising the above.]

* [http://www.osi.ie/en/alist/reference-information.aspx?article=42177d93-831a-4c6c-8ebf-cef57b55cd93 History and overview of the system, from OSI]
* [http://www.osi.ie/en/alist/co-ordinate-converter-tool.aspx OSI online coordinate converter tool] .(If "registration" is required enter: "[e-mail]" for the email address and "password" for the password.)
* [http://www.gridreference.ie Clickable webmap that shows Irish grid references]